1. Field of the Invention
The present invention relates to an optical material and an optical element, and more particularly, to an optical material suitable to form an optical element used for an image pickup optical system of a camera.
2. Description of the Related Art
Up to now, an example of methods of correcting chromatic aberration of an optical system including only a refraction system is a method using a combination of glass materials having different dispersion properties. For example, an objective lens of a telescope includes a positive lens made of a glass material whose Abbe constant (νd) is large and a negative lens made of a glass material having a small Abbe constant. A combination of the positive lens and the negative lens is used to correct axial chromatic aberration. Therefore, when a lens structure or the number of lenses is limited or when used glass materials are limited, the chromatic aberration may not be sufficiently corrected. In order to correct such chromatic aberration, there has been known a method of controlling a refractive index and an Abbe constant to obtain, for example, a glass material having a high refractive index and a low Abbe constant.
In particular, U.S. Pat. No. 5,847,877 discloses that a heat-curable resin or a light-curable resin which is excellent to obtain a desired shape by the application of heat or light or a thermoplastic resin for extrusion molding has been used as an optical material including an organic compound.
In addition, U.S. Pat. No. 6,759,471 discloses that an organic-inorganic hybrid optical material having desired optical properties, which has been developed with the current progress of nanotechnology. According to the organic-inorganic hybrid optical material, ultra-fine particles of several nm to several tens nm are dispersed in a resin without being coagulated. Therefore, an optical material exhibiting optical properties which may not be realized by existing glass may be adjusted.
When an optical element which is excellent in chromatic aberration correction function and has an aspherical shape is to be manufactured, a case where a light-curable resin, a heat-curable resin, or a thermoplastic resin is molded on a spherical glass used as a base is more excellent in mass productivity, processability, and moldability than a case where an optical glass is used as a material. However, a plastic resin and a curable resin which normally each have a high refractive index and a small Abbe constant cause yellowing. That is, the resins cause yellowing because of a fundamental molecular structure, resin decomposition due to heating during processing or irradiation of energy such as ultraviolet light, or a change in molecular structure during reaction. An organic optical material used for the optical element is required to satisfy optical performance and at the same time to be more transparent.
In recent years, an organic-inorganic hybrid optical material has been proposed in which ultra-fine particles such as metal oxide particles are dispersed in an organic resin to mainly change optical properties. However, in order to improve a resin having general properties to have a desired property, an optical design is required to add a greater amount of fine particles and uniformly disperse the fine particles. When a greater amount of fine particles is added and the dispersibility of the fine particles is low, a transmittance reduces and an optical scattering property deteriorates. In a case of a hybrid material, as a refractive index difference between the organic resin and the fine particles increases, the harmful influence on the optical scattering property becomes larger, and hence it is said to be desirable to minimize the refractive index difference.
In contrast to this, the inventor of the present invention found that not only the high refractive index and the low Abbe constant but also a secondary dispersion property (θg,F) are important as material properties for providing the optical element with the chromatic aberration correction function. That is, a material having a secondary dispersion property larger than a general material (refractive index extraordinary dispersion property) is very effective for chromatic aberration correction in optical design.
FIG. 1A is a graph illustrating a relationship between the Abbe constant νd and the secondary dispersion property θg,F in glass materials commercially available as optical materials. In FIG. 1A, the ordinate indicates the secondary dispersion property θg,F and the abscissa indicates the Abbe constant νd. Normal optical glass materials have a property substantially on the line expressed by the following (Expression 1).θg,F=0.6438−0.001682νd  (Expression 1)
FIG. 1B is a graph illustrating a relationship between the Abbe constant νd and a refractive index nd in materials commercially available as optical materials. In FIG. 1B, the ordinate indicates the refractive index nd and the abscissa indicates the Abbe constant νd.
In FIGS. 1A and 1B, optical materials having secondary dispersion properties larger than the secondary dispersion property obtained by (Expression 1) described above are expressed by a white square. Specifically, there are Vinylcarbazole (produced by Tokyo Chemical Industry Co., Ltd.) (nd=1.69, νd=17.9, and θg, F=0.70), UV1000 (produced by Mitsubishi Chemical Corporation) (nd=1.63, νd=23.3, and θg,F=0.67), HV153 (produced by ADELL Corporation) (nd=1.63, νd=25.0, and θg,F=0.653), and MPV (produced by Sumitomo Seika Chemicals Co., Ltd.) (nd=1.70, νd=17.4, and θg,F=0.71).
However, Vinylcarbazole may not be desirable as an optical material because of significant yellow coloring (yellowing) and a low transmittance. In addition to this, Vinylcarbazole has high brittleness and is likely to cause cracks during film formation, to thereby reduce moldability. UV1000 has a high transparent property, but a material cost thereof is extremely high and general versatility is low. HV153 is more transparent than Vinylcarbazole, but a material cost thereof is high and general versatility is low. A material cost of MPV is low, but a transmittance thereof is not sufficient for an optical element. Therefore, a single resin has not been found as a desired optical material having general versatility.
Comparisons among the physical properties and material costs of the four optical materials described above are illustrated in Table 1.
TABLE 1Optical property(exhibiting θg, FCommerciallyequal to or largeravailable opticalthan that obtainedBrittlenessMaterialTotalmaterialsby Expression 1)Transmittance(crack)costdeterminationVinylcarbazoleGoodNGNGGoodNGUV1000GoodGoodGoodNGNGHV153GoodGoodGoodNGNGMPVGoodNGGoodGoodNG